If yo are really worried about this just use a plywood/fiberglass/plywood "Sandwich" instead of just a piece on the inside of the cabinet.
Then use wide headed bolts outside and T-Nuts on the inside and this will not budge at all.
I did this on 2 Trillium
that I have had and it worked out great on both.
I use mainly Baltic Birch and I think 1/2" but don't remember that detail well either?
Ease the edges and poly or stain to match the rest of the rig and then bolt thru both pieces and you wall can not flex as the wood will make the wall rigid where it is.
I made mine a lot bigger than it needed to be but back then TVs were a lot heftier than they are now too!
Also just so you know,I hang TVs for a living every single day and am obsessed with worry free mounting on every install,EVERY INSTALL!